U.S. patent number 5,917,856 [Application Number 08/849,010] was granted by the patent office on 1999-06-29 for method for establishing a pam signal connection using a training sequence.
This patent grant is currently assigned to Nokia Telecommunications Oy. Invention is credited to Simo-Pekka Torsti.
United States Patent |
5,917,856 |
Torsti |
June 29, 1999 |
Method for establishing a PAM signal connection using a training
sequence
Abstract
In a method for establishing a data communication connection in
a data channel utilizing PAM signals, a transmitter transmits a
training sequence, formed of predetermined symbols, a predetermined
number of times. The data channel is monitored in order to detect a
predetermined signal, samples are taken and amplification is
adjusted in such a way that the signal, settle at a predetermined
level. In order that the method can be applied in a data channel
with previously unknown properties, when the transmitter starts
repeating the training sequence, samples taken from the data
channel are buffered to form a sample sequence. A channel pulse
response estimate is calculated by convoluting the buffered samples
with a predetermined symbol sequence. A main sample is selected,
correction coefficients are calculated on the basis of the samples
preceding and following the main sample, and, when the transmitter
starts transmitting data, the samples taken from the data channel
are corrected by utilizing the correction coefficients.
Inventors: |
Torsti; Simo-Pekka (Espoo,
FI) |
Assignee: |
Nokia Telecommunications Oy
(Espoo, FI)
|
Family
ID: |
8541701 |
Appl.
No.: |
08/849,010 |
Filed: |
April 30, 1997 |
PCT
Filed: |
October 30, 1995 |
PCT No.: |
PCT/FI95/00598 |
371
Date: |
April 30, 1997 |
102(e)
Date: |
April 30, 1997 |
PCT
Pub. No.: |
WO96/13908 |
PCT
Pub. Date: |
May 09, 1996 |
Foreign Application Priority Data
Current U.S.
Class: |
375/231; 370/292;
375/353 |
Current CPC
Class: |
H04L
25/0224 (20130101); H04L 25/0212 (20130101); H04L
25/4917 (20130101); H04L 25/03292 (20130101); H04L
27/02 (20130101) |
Current International
Class: |
H03H
7/30 (20060101); H04L 29/00 (20060101); H04L
7/033 (20060101); H04B 3/23 (20060101); H03H
007/30 () |
Field of
Search: |
;375/231,222,346,353
;370/286,292 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
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|
|
|
|
228771 |
|
Jul 1987 |
|
EP |
|
0368189 |
|
May 1990 |
|
EP |
|
373277 |
|
Jun 1990 |
|
EP |
|
384490 |
|
Aug 1990 |
|
EP |
|
391715 |
|
Oct 1990 |
|
EP |
|
0403716 |
|
Dec 1990 |
|
EP |
|
573696 |
|
Dec 1993 |
|
EP |
|
2 198 015 |
|
Jun 1988 |
|
GB |
|
Other References
Benndorf, J. et al.: "Application of Fast convolution Algorithms
Using Sampling Frequency change in Digital High-Speed
Modems"..
|
Primary Examiner: Ghebretinsae; Temesghen
Attorney, Agent or Firm: Pillsbury Madison & Sutro
LLP
Parent Case Text
This application is a national phase of international application
PCT/FI95/00598, filed Oct. 30, 1995 which designated the U.S.
Claims
I claim:
1. A method for establishing a data communication connection in a
data channel where pulse amplitude modulation signals are utilized,
a transmitter transmitting a training sequence formed of
predetermined symbols, a predetermined number of times, said method
comprising:
a receiver monitoring the data channel in order to detect a
predetermined signal;
taking samples from the data channel and adjusting amplification of
the receiver in such a way that signals received by the receiver in
consequence of transmissions by the transmitter settle at a
predetermined level;
when the transmitter, in transmitting said training sequence,
starts repeating the training sequence during a time period it
takes to transmit one training sequence, buffering the samples
taken from the data channel in said taking in order to form a
sample sequence;
calculating a channel pulse response estimate by convoluting the
buffered samples with a predetermined symbol sequence;
selecting a main sample from said pulse response estimate such that
others of said samples precede and follow said main sample;
calculating correction coefficients from the pulse response
estimate on the basis of the samples preceding and following the
main sample, in order to minimize interference on the data channel
and
when the transmitter stops transmitting the training sequence (t)
and starts transmitting data, correcting the samples taken from the
data channel by utilizing said correction coefficients.
2. A method according to claim 1, wherein:
the transmitter is a modem in a multipoint network and the data
channel is a copper cable which, in said taking of samples, is
sample twice during each symbol.
3. A method according to claim 1, wherein:
when the energy contained in the samples (t') taken from the data
channel exceeds a predetermined threshold level for the first time,
in consequence thereof determining that said predetermined signal
has been received; and
starting a timer means to measure the time period it takes to
transmit the training sequence, thereby determining the moment the
transmitter starts repeating the training sequence.
4. A method according to claim 1, wherein:
in said calculating, a sequence which forms, when convoluted with
the training sequence, a sequence having one element with an
approximate value of one and other elements having values
approximating to zero, is selected as said predetermined symbol
sequence.
5. A method according to claim 1, wherein:
in said selecting, the main sample of the pulse response estimate
is selected from the sample sequence in such a way that all the
samples having a higher absolute value than a respective following
and a respective preceding sample, when the sample sequence is
viewed as recurrent, are selected preliminarily, and the sample
with a longest interval to the preceding preliminarily selected
sample, when the sample sequence is considered as recurrent, is
selected from the selected samples as the main sample.
Description
BACKGROUND OF THE INVENTION
The invention relates to a method for establishing a data
communication connection in a data channel where PAM signals are
utilized, the transmitter transmitting a training sequence, formed
of predetermined symbols, a predetermined number of times, in which
method: the data channel is monitored in order to detect a
predetermined signal, samples are taken from the data channel, and
the amplification of the receiver is adjusted in such a way that
the received signals settle at a predetermined level.
The invention relates especially to multipoint modem networks
utilizing PAM (Pulse Amplitude Modulation) signals and comprising
one master modem which transmits the same data to several slave
modems. The slave modems alternately transmit data to the master
modem by reserving the channel in turn for their own use. In
addition to transmission rate, training time also significantly
affects the amount of data that can be transmitted in a time unit
in such a network. In this connection, training time refers to the
time required for establishing a connection. During the training
time, synchronization is performed with the remote clock, and a
descrambler and amplification are set, among other things.
In a previously known arrangement for establishing a
datacommunication connection, fixed correction coefficients,
dependent on the data channel used, are determined beforehand for
the receiver, and, by means of the coefficients, the interference
resulting from the data channel, between the symbols to be
transmitted, is compensated for. A typical known equalizer of this
type is a FIR (Finite Impulse Response) equalizer by means of which
both the front part and the tail part can be removed from a pulse
response. Since the correction coefficients are fixed, a connection
can be established by setting correct amplification, synchronizing
the receiver with the transmitter clock, and by setting a
descrambler.
The major drawback of the aforementioned known solution is that
when it is applied, the pulse response of the data channel must be
known, so that the fixed correction coefficients can be set
correctly. For example, when a multipoint network is set up, the
assembler must manually adjust the correction coefficients of each
modem to a suitable value. This is due to the fact that, for
example when a copper cable is used, the pulse response of the data
channel changes according to the cable length, so that the
correction coefficients must always be set specifically for each
case.
SUMMARY OF THE INVENTION
The purpose of the present invention is to eliminate the
aforementioned drawback and to provide a method of establishing a
datacommunication connection, which method requires no manual
adjustments of the type described above. This object is achieved
with the method according to the invention, characterized in that
when the transmitter starts repeating the training sequence, during
a time period it takes to transmit one training sequence the
samples taken from the data channel are buffered in order to form a
sample sequence, a channel pulse response estimate is calculated by
convoluting the buffered samples with a predetermined symbol
sequence, a main sample is selected from the pulse response
estimate, correction coefficients are calculated from the pulse
response estimate on the basis of the samples preceding and
following the main sample, in order to minimize the interference on
the data channel. And when the transmitter stops transmitting the
training sequence and starts transmitting data, the samples taken
from the data channel are corrected by utilizing the correction
coefficients calculated.
The invention is based on the idea that when the receiver is
adapted to calculate the pulse response estimate, on the basis of a
predetermined training sequence transmitted through the data
channel, by convoluting samples taken from the received training
sequence with a predetermined sample sequence, and when the
correction coefficients are set on the basis of the calculated
pulse response estimate, there is no need to preadjust the receiver
according to the properties of the data channel to be used. When
the method according to the invention is applied, the receiver can
itself detect the properties of the data channel and select the
correction coefficients suitable for the data channel in question.
The major advantages of the True Fast Poll method according to the
invention are therefore that the method can be applied in data
channels with an unknown pulse response, and that a
datacommunication connection can be established within a very short
time by means of the method. For example, when a 2B1Q signal is
used, the receiver can be prepared for data transmission within a
time required for transmitting 60 symbols (when the length of the
correctors is 20 symbols). When the pulse response estimate is
calculated with the method according to the invention, the rotation
of the training sequence perceived by the receiver is
insignificant, since the possible rotation can be compensated for
during the selection of the main sample.
In the following, the invention will be described in greater detail
by means of a preferred embodiment of the method according to the
invention, with reference to the accompanying drawings, in
which:
FIG. 1 is a flow chart of the method according to the invention,
and
FIG. 2 illustrates the selection of the main sample from the pulse
response estimate.
DETAILED DESCRIPTION
FIG. 1 is a flow chart of a preferred embodiment of the method
according to the invention, applicable in establishing a data
communication connection, for example in a multipoint modem network
utilizing PAM signals, such as 2B1Q signals, and employing a copper
cable as a data channel. However, it should be noted that the
method according to the invention can also be utilized in other
connections, for example in a radio channel or in an optical data
channel.
In the case of FIG. 1, the transmitter can be adapted to repeat
three times a training sequence of 20 symbols, after which the
transmitter starts transmitting data. The receiver knows beforehand
the symbols included in the training sequence, and the number of
times the training sequence is repeated. The training sequence is a
predetermined symbol sequence containing no DC, and having a
spectrum that is as even as possible, and an energy amount
corresponding as accurately as possible to the average energy of
symbols transmitted in the actual data transmission, so that the
amplification can be adjusted to the correct level in the receiver.
The length of the training sequence used must be such that the
pulse responses that must be corrected are shorter in time than the
time it takes to transmit the training sequence.
In method step A, the receiver is adapted to monitor the data
channel in order to detect a predetermined signal. The receiver can
be arranged, for example, to sample the data channel twice during a
symbol.
In method step B, the energy of the samples from the data channel
is examined. The detection of energy from the line can be utilized
as the predetermined signal. The receiver remains in the wait
condition until energy arrives from the data channel after the
transmitter has started transmitting the training sequence t. The
receiver detects this in such a way that the energy contained in
the samples exceeds a predetermined threshold E.sub.0.
In method step C, the amplification of the receiver is adjusted to
a suitable level, so that an A/D converter provided in the receiver
can sample the line with a sufficient accuracy.
In method step D, the receiver waits until the transmitter starts
repeating the training sequence t. Since, the receiver is aware of
the length of the training sequence transmitted by the transmitter,
there are two ways of finding out the moment the training sequence
will be repeated:
calculating correlation for signals arriving from the line, or
measuring time from the moment energy was first detected on the
line. When the time required for transmitting the training sequence
has elapsed since the time the energy increase was detected, the
transmitter starts repeating the training sequence.
However, when the method according to the invention is applied,
there is no need to determine the starting point of the second
training sequence with an accuracy of a symbol, but it is
sufficient that method step E will not be started until it is
certain that the first training sequence is over (the second or
third training sequence must be used when the pulse response
estimate is calculated, so that the tails of the preceding training
sequence can be made visible).
In method step E, the receiver is adapted to sample the line for
the time it takes to transmit the training sequence t, and to
buffer the samples taken. The buffer will thus contain a sequence
t' formed of forty samples, if the length of the training sequence
is 20 characters and the sampling frequency is 2 samples/symbol. If
the moment the sampling of the line begins does not correspond
exactly to the starting point of the training sequence, it means
that the sample sequence gathering in the buffer is slightly
rotated (i.e. the first buffered sample has been taken, for
example, from the third symbol of the training sequence, etc.)
However, this does not cause problems in the application of the
method of the invention, since according to the invention, the
transmitter is arranged to repeat the training sequence more than
once so that the sequence gathering in the buffer contains samples
of all the symbols of the training sequence, even though the
buffered sequence may be rotated. Possible rotation does not
require additional operations (since such rotation will be
compensated for when the main sample is selected), wherefore the
invention will be described below assuming that the sequence t' is
not rotated, but the first and the second (if samples are taken
twice during a symbol) buffered sample have been taken from the
first symbol of the training sequence.
In method step F, the pulse response estimate of the data channel
is calculated by means of the buffered samples. When a training
sequence t has been transmitted through the data channel h, a
sequence t'=t*h (*=convolution) is formed in the buffer of the
receiver. When the buffered sequence t' is convoluted with a
predetermined sequence r, the following sequence is received,
r*t'=r*(t*h)=(r*t)*h=k*h, i.e. the channel impulse response
convoluted with the sequence k. If the sequence r is suitably
selected, the sequence k is a pulse supplemented with DC, wherefore
if the sequence h does not contain DC, the convolution k*h will
result in h, i.e. the channel pulse response.
The selection of the aforementioned predetermined sequence r
depends entirely on the selection of the training sequence t. A
possibility of selecting the training sequence t and the
aforementioned predetermined sequence r is illustrated in the
following table:
______________________________________ t(n) r(n) k(n) = t(n)*r(n)
______________________________________ 3 0.038688 -0.05 1 -0.02839
0.95 -1 -0.01401 -0.05 1 -0.02245 -0.05 1 -0.02169 -0.05 -3
0.033688 -0.05 1 0.013943 -0.05 -1 -0.00848 -0.05 3 0.00036 -0.05
-1 0.029305 -0.05 3 0.030066 -0.05 3 -0.00631 -0.05 -1 0.036667
-0.05 -1 -0.01183 -0.05 1 0.023045 -0.05 3 -0.02542 -0.05 -3
0.011836 -0.05 -3 0.019051 -0.05 -3 -0.00641 -0.05 -3 0.008344
-0.05 ______________________________________
It can be seen from the table that when the recurrent training
sequence t is convoluted with the sequence r, i.e. ##EQU1## (where
N=length of the training sequence t), the result is sequence k
consisting of a value one and of n-1 other values (that approximate
to zero).
In other words, when the training sequence t has been transmitted
through the data channel h, it is visible in the buffer of the
receiver as the sequence t', and when this sequence is convoluted
with a predetermined sequence r, the result is the sequence
r*t'=r*(t*h)=(r*t)*h=k*h, which is the channel pulse response h, if
h does not contain DC.
If samples are taken twice during a symbol and the length of the
training sequence t is 20 symbols, the length of the buffered
sequence t' is 40 elements. Therefore the length of the
predetermined sequence r must also be 40 elements. This can be
implemented in such a way that every other element in the sequence
r is taken from the table above, and zero is set as the value of
every other element (i.e. r=[0.038688, 0, -0.02839, 0, 0.01401, 0,
. . . ]).
In method step G, the main sample is selected from the pulse
response estimate. This can be performed for example, in such a
manner that all the absolute values of the sequence h are
calculated first, whereafter the main sample is selected from among
those samples that have a higher absolute value than the absolute
value of the preceding and the following sample. In other words,
all the local maximums of the absolute values of the sequence are
selected preliminarily. The main sample will be the sample with the
longest interval to the preceding maximum when the sequence is
viewed as recurrent (i.e. h(n)=h(n+N), wherein N is the number of
elements in the sequence). If the absolute value of the element
preceding the candidate for the main sample in the sequence is less
than 10% lower than the absolute value of the main sample
candidate, this element preceding the candidate for the main sample
is selected as the main sample. When the main sample is set, the
amplification of the receiver is checked and, if necessary,
adjusted to a suitable level.
In method step H, the necessary correction coefficients are
calculated on the basis of the pulse response estimate h in order
to minimize the interference. The manner of calculating the
correction coefficients depends entirely on what kind of filters
are used in the reception. For example, if a three-corrector FIR
equalizer is utilized, in a manner known per se, for removing the
leading samples preceding the main sample (for example two elements
preceding the main sample in the sequence), the correctors are
obtained directly from the value of the leading samples of the
calculated pulse response estimate. If a decision feedback
equalizer (DFE) is utilized in a manner known per se in the
receiver to remove the tail (the samples following the main sample
in the pulse response estimate), the coefficients are
correspondingly obtained from the tail of the pulse response h,
which, however, must first be treated with the FIR. When the DFE
coefficients have been calculated and before the DFE is taken into
use, the delay line of the DFE must be set. The DFE delay line must
comprise the symbols that have been received before that moment.
The symbols can be deduced from the place of the main sample
candidate.
In method step I, the receiver is set in a wait condition until the
transmitter has repeated the training sequence for the last (third)
time, after which data reception is started. The moment the last
symbol of the last training sequence will be received is determined
by counting symbols from the sampling moment the main sample
candidate was found. When the last symbol of the last training
sequence has been received, the receiver sets the descrambler to a
value corresponding to the value of the scrambler in the
transmitter.
In method step J, the transmitter receives data by sampling the
data channel, after which the samples are corrected with the
calculated correction coefficients before decisions are made based
on the samples. In all the method steps, it is made sure that the
receiver receives energy from the line. If even a slight energy
breakdown is detected on the line, the transmitter proceeds to the
condition according to process step A to wait for the reception of
a new training sequence.
FIG. 2 illustrates the selection of the main sample from the pulse
response estimate h (method step G). First, all the absolute values
of the sequence h are calculated, whereafter the elements having a
higher absolute value than the absolute value of the preceding
element and the following element are preliminarily selected from
the sample sequence. In other words, all the local maximums of the
absolute values of the sample sequence are preliminarily selected.
In the case of FIG. 2, samples 2 and 7 are preliminarily
selected.
The sample having the longest interval to the preceding maximum,
when the sequence is viewed as recurrent as shown in FIG. 2, is
selected as the main sample. In the case of FIG. 2, the interval
from sample 2 to the preceding sample, i.e. to sample 7, is fifteen
units, and from sample 7 to the preceding sample, i.e. to sample 2,
five units. Therefore, sample 2 is selected as the candidate for
the main sample.
In the following, sample 1 preceding sample 2 will be observed. If
the absolute value of sample 1 preceding the main sample candidate
2 is less than 10% lower than the absolute value of the main sample
candidate 2, this preceding sample is selected as the main sample.
In the case of FIG. 2, the aforementioned requirement is not
fulfilled, however, wherefore sample 2 is selected as the main
sample. In the case of FIG. 2, the front part of the pulse response
therefore contains only one sample, i.e. sample 1. The tail
correspondingly consists of samples 3 to 20.
It should be understood that the above description and the drawings
related thereto are only intended to illustrate the present
invention. Different variations and modifications of the invention
will be evident for a person skilled in the art, without departing
from the scope and spirit of the invention disclosed in the
appended claims.
* * * * *